An unpredictable, yet scorching and dry climate kept large, grass-eating dinosaurs out of the tropics for some 30 million years after they first appeared on Earth, a study out Monday found.

It has been a longstanding mystery: why did long-necked dinosaurs seem to avoid the tropics when there were many different types of them lived at different latitudes well north and south of the equator? There are fossil remains, however, of meat-eating dinosaurs in the tropics.

Scientists working at a site in northern New Mexico worked with rocks from 215 to 205 million years ago and were able to recreate the climate from that time, the Late Triassic Period.

They found that the tropical climate varied greatly, but was usually too hot for plants, and as such for herbivores, to thrive.

The carbon-loaded atmosphere had humid phases and then long droughts with temperatures of about 600 degrees.

"Our data suggest it was not a fun place," study co-author Randall Irmis of the University of Utah said in a statement.

"It was a time of climate extremes that went back and forth unpredictably. Large, warm-blooded dinosaurian herbivores weren't able to exist close to the equator -- there was not enough dependable plant food."

An international team of researchers led by geochemist Jessica Whiteside had their findings published in the journal Proceedings of the National Academy of Sciences.

"These scientists have developed a new explanation for the perplexing near-absence of dinosaurs in late Triassic equatorial settings," said Rich Lane, program director in the National Science Foundation's Division of Earth Sciences, which funded the research.

"That includes rapid vegetation changes related to climate fluctuations between arid and moist climates and the resulting extensive wildfires of the time."

During the period studied, atmospheric carbon dioxide levels then were four to six times current levels.

"If we continue along our present course, similar conditions in a high-CO2 world may develop, and suppress low-latitude ecosystems," Irmis said.